Integrated circuit design and fabrication is a vastly complex effort and involves interaction between numerous steps in a manufacturing process. While going through the various steps in the design process, an understanding is required of the limitations of the manufacturing process. Shapes must be designed which can be fabricated so that the desired electronic circuit function is implemented at the resulting end of fabrication. Frequently, millions and even hundreds of millions of transistors can exist on a single semiconductor chip. Each transistor is composed of shapes for diffusion, polysilicon, contacts, metallization, and other structures. The ability to design chips with such large numbers of essential transistors can be quite challenging and optimization of the circuitry as needed can be a daunting task even with the help of electronic design automation (EDA) software tools.
There are numerous metal lines of miniscule dimension in close proximity to one another on each semiconductor chip. Further, there are diffusions, polysilicon shapes, and insulator layers, all of which must be fabricated to exacting tolerances. As technologies have advanced, smaller and smaller dimensions are used in lithography. These smaller dimensions allow for smaller defects to have a greater impact. A defect can impact a circuit by bridging between two structures, thereby causing a short or resistive short, or even by increasing capacitance between adjacent structures. Defects can enter a semiconductor chip at each step in the fabrication process. No amount of effort will ever eliminate defects from the manufacturing process. Further, failures on semiconductor chips may be the result of random defects or systematic defects on the chips.